Linear gyrokinetic stability of a high ß non-inductive spherical tokamak

Spherical tokamaks (STs) have been shown to possess properties desirable for a fusion power plant such as achieving high plasma ß and having increased vertical stability. To understand the confinement properties that might be expected in the conceptual design for a high ß ST fusion reactor, a 1 GW ST plasma equilibrium was analysed using local linear gyrokinetics to determine the type of micro-instabilities that arise. Kinetic ballooning modes and micro-tearing modes are found to be the dominant instabilities. The parametric dependence of these linear modes was determined and, from the insights gained, the equilibrium was tuned to find a regime marginally stable to all micro-instabilities at ? 0 = 0.0. This work identifies the most important micro-instabilities expected to generate turbulent transport in high ß STs. The impact of such modes must be faithfully captured in first-principles-based reduced models of anomalous transport that are needed for predictive simulations.